The present invention relates to improved solar heating arrangements, and is more particularly concerned with a solar collector adapted for use in a system of the type described in my prior copending application Ser. No. 665,648 filed Mar. 10, 1976, for Closed System Solar Heater, now U.S. Pat. No. 4,010,734, issued Mar. 8, 1977, the disclosure of which is incorporated herein by reference.
Various forms of solar heating system have been suggested heretofore which use energy from the sun as a source of heat. In general, such systems include a solar collector structure arranged to permit the flow of an appropriate medium, such as water, therethrough and adapted to cause said medium to be heated as it flows through the collector due to solar radiant energy incident on the collector; and the solar collector structure in turn forms part of an overall solar heating system arranged to permit immediate use of the heated medium, or adapted to provide storage of the heated medium for later utilization.
A wide variety of solar collector structures have been suggested heretofore. In some cases, the collector comprises a continuous length of tubing through which water may flow, with the tubing being coiled into a variety of configurations e.g., as shown in Gould U.S. Pat. No. 1,747,826, Trombe U.S. Pat. No. 2,552,237, Fox U.S. Pat. No. 2,978,562, Garrett U.S. Pat. No. 3,200,820, Thomason U.S. Pat. No. 3,215,134, Thannhauser U.S. Pat. No. 3,254,644, and Long U.S. Pat. No. 3,778,578. One comparatively accepted collector of the tubing type employs a tubular array in association with a superposed metallic plate which operates to heat water flowing through the tubing in a three-step transfer process, i.e., the plate is first heated by incident radiant energy, with the heated plate in turn heating the tubing, which then heats water flowing through the tubing. This technique requires successive heat transfer steps which have been found in practice to be somewhat inefficient. As will appear hereinafter, the collector of the present invention, by arranging the tubing itself so that it acts as a radiant energy adsorptive plate, eliminates one of the aforementioned heat transfer steps and similarly eliminates a related source of energy loss, with a resultant significant increase in efficiency.
In tubing-type solar collectors which have been suggested heretofore, the actual quantity of water which passes through the array, and the temperature to which it is heated, depends upon the dimensions of the tubing used in the array. It has been the standard practice heretofore to employ tubing of a single diameter throughout the array and, in such an arrangement, an increase in the tubing diameter tends to achieve a greater quantity of water flow through the array, but effects less heat per unit volume of flow. Conversely, a decrease in the diameter of the tubing, when uniform diameter tubing is used throughout, decreases the quantity of flow but increases the temperature to which the water is heated and, indeed, the reduction in tubing diameter can be such that the solar collector produces sufficient heat to convert the flowing water to steam, a result which may be desirable for some applications but which is undesirable when the collector is to be employed as part of a residential water heating system. In an effort to achieve an adequate flow rate and an adequate temperature rise, it has been the practice heretofore in connection with tubing-type solar collectors to select the tubing diameter and tubing length in the collector in a fashion which achieves a compromise between these two factors. However, since it has been customary to use tubing of a single diameter throughout the array, the result of the compromise has been less than entirely satisfactory in relation to one factor or the other. Where adequate flow rates have been achieved, the total temperature rise is usually too small; and where an adequate temperature rise has been achieved the total volumetric flow is often insufficient to satisfy normal residential requirements.